Many applications using communication networks, for example Voice over IP (VoIP), are sensitive to traffic loss, such as occurs when a link or router in the communication network fails.
In communication networks a feature known as Loop-Free Alternate (LFA) has been developed to provide a back-up path in the event that a node or link in the communication network becomes faulty or non-operational. LFA effectively performs a form of local repair, whereby a router local to a point of failure is preprogrammed with an alternate next hop, which is activated when a failure is detected. The loop-free local repair is active until a global repair route is activated, for example until a distributed network convergence process completes (this being a form of global repair).
The International Engineering Task Force (IETF) request for comments RFC5286 provides further details of the Basic Specification for IP Fast Reroute: Loop-Free Alternates, and offers a method for calculating loop-free alternates based on a calculation of route inequalities.
In a communication network adopting a LFA mechanism for a protected link, the term “P-space” relates to a set of routers reachable from a specific router without any path (including equal cost path splits) transiting the protected link. The term “Q-space” relates to a set of routers from which a specific router can be reached without any path (including equal cost path splits) transiting the protected link.
An extension to LFA is known as remote LFA (RLFA). Remote LFA is a repair mechanism that provides additional backup connectivity for link failure protection when none can be provided by the basic mechanism. It fills the gap when a link cannot be fully protected by a traditional local LFA neighbour.
When remote LFA is applied for link failure protection, especially in ring based topologies, a Target Label Distribution Protocol (TLDP) session will be generated and stay resident between a Point of Local Repair node (PLR node) and a PQ node. A PQ node is a member of both the extended P-Space and the Q-space (extended P-Space being the union of P-space of the neighbours of a specific router with respect to the protected link). In remote LFA this PQ node is used as a repair tunnel endpoint node.
Instead of switching to another available Label Switched Path (LSP) directly from a source point, packets will first follow the original path to the PLR node, then switch to a PQ node through a pre-calculated backup LSP, as will be described below in relation to FIGS. 1 and 2. The back-and-forth forwarding path has disadvantages such as occupying a portion of available link bandwidth, and increasing the recovery time after a failure, and is thus not an optimal switchover path.
FIG. 1 shows an example of a typical ring based topology comprising seven nodes 101 to 107. If a path is to be established from node 101 to node 105 (node 101 being the source node and node 105 being the destination node in such an example), a shortest path will result in packets entering from node 101 being routed via node 107 and node 106 to node 105.
If a link failure or switchover event (for example because of a degradation of a link) occurs between node 106 and node 105, as shown in FIG. 2, then node 106 will effectively become the Point of Local Repair, i.e. PLR node. The node 106 will set up a targeted LDP (TLDP) session 13 with a repair tunnel endpoint node (the PQ node), this being node 102 in the example of FIG. 2. Further details about how the PQ node is determined may be found in a IETF paper referenced <draft-ietf-rtgwg-remote-lfa-04>. The PLR node is pre-configured before link failure or degradation, as is the PQ node and the TLDP session between the PLR node and the PQ node. This TLDP session needs to be established and the labels processed before the tunnel can be used.
When a link failure occurs the PLR node first needs to swap the original top label stack to the label stack used by the repair tunnel endpoint (PQ node) to the destination, and then push its own label for the repair tunnel endpoint (PQ node). Remote LFAs operating in this manner preserve the benefits of RFC5286, which include simplicity, incremental deployment and good protection coverage.
However, packets entering the source node 101 will continue along the original path to node 106, where they will turn around and follow an opposite side LSP path towards node 101, and then via node 102 to node 103, from node 103 to node 104, and from node 104 to node 105.
The packets take the back-and-forth forwarding path when switchover occurs to another available path, and the packet source node (node 101) does not have any sense of the link failure, since all the switchover procedures are carried out by the PLR node, i.e. node 106 in this example. The unnecessary forwarding direction has the disadvantage of occupying twice the link bandwidth for its back-and-forth behavior, and this waste of bandwidth is increased when there are more nodes between the PLR node (node 106) and the packet source node (node 101).
In addition to such bandwidth disadvantages, another disadvantage is that of LDP convergence time. Known remote LFA switchover mechanisms mainly depend of Interior Gateway Protocol (IGP) convergence, which can take several seconds in a large network.